An EL indicating panel generates electro-luminescence when an electric field is applied to a film composed of zinc sulfide with small amounts of some transition metal such as Mn and rare earth elements. EL indicating panels have been known and used for quite some time and generally consist of a thin film construction composed of a light emitting film made of the above electro-luminescence materials and an insulating film making contact therewith either from both sides or one side being sandwiched between a lower electrode film and an upper electrode film. (Refer to, for example, M. J. Russ, et al; J. Electrochem. Soc. 114, 1964, p 66, and T. Isoguchi, et al; '74 SID Intern. Symp. 1974, pp. 84-85.) Furthermore, EL indicating panels-with luminance arranged with many fine EL picture elements on the panel surface have recently become available, and they are being watched closely for their future potential as self-lighting flat panel suitable for variable images. Explanations will be given hereunder of representative examples of such EL indicating panels with thin-film construction through reference to FIG. 5.
FIG. 5 shows a partially expanded view of a substrate for the EL panel that normally uses a transparent insulation substrate (10) made of glass or similar material over which many extremely thin electrode films, measuring about 2000.ANG. and made of ITO (indium tin oxide), are disposed as the lower electrode film (20). The lower electrode film (20) is formed in a striped pattern that is long and narrow from front to back and is arranged from right to left in a very narrow pitch of 100-150 .mu.m, so that about eight films are usually laid within a distance of 1 mm. Then on top of this, an insulating film (30) with a thickness of about 3000.ANG. made of a high insulation material, such as silicon nitride, a light emitting film (40) with a thickness of about 5000.ANG. such as ZnS containing Mn, and an insulating film (50) which is identical to the insulating film (30) are laminated in that order. Next, many upper electrode films (60) with a thickness of about 5000.ANG., and made of a metal such as aluminum, are disposed as the uppermost layer in such a manner that they perpendicularly intersect with the lower electrode films (20) in a long and narrow striped pattern from right to left as shown in the figure, and in an identically narrow arranging pitch from front to back as shown in the figure.
Thus, the EL indicating panel has a light emitting film (40) sandwiched by the insulating films (30) and (50) between the lower electrode films (20) and upper electrode film (60), while the parts of the light emitting film (40) corresponding to the intersection area of the striped pattern in which the electrode film (20) and (60) intersect with each other constitute the picture elements for indication. All the parts in the EL indicating panel are transparent except for the upper electrode film (60). The indicating voltage is applied across the electrode films (20) and (60) so that an electric field will be applied to the light emitting film (40) via the insulating films (30) and (50). Furthermore, the electro-luminescence generated from atoms of Mn or some similar element contained in the ZnS acting as the light emitting cores will be taken out from the insulating substrate (10) as an indicating light "LD" as shown in the figure.
Because the light emitting film (40) is an insulator, both of its sides may not necessarily be sandwiched by the insulating films, and it is therefore possible to omit either one of the insulation film (30 or 50) to minimize the film thickness sufficiently to prevent atom migration which is harmful to the light emitting film (40). In addition, the insulation film must be made as thin as possible to reduce the indication voltage and raise light emitting efficiency of the light emitting film. Meanwhile, the above described film thickness allows the internal electric field strength to normally reach about 10.sup.6 V/cm for an indication voltage of about 200 V.
There is a problem with insulation reliability in EL indicating panels having internal electric field strength in the insulating films as high as described above, and the displayed image quality tends to be degraded while the service life is shortened as a result of local insulation breakdown which develops at various weak points on the insulation films. For instance, the weak points on the insulation are the parts, "A", as referred to the lower insulation film (30) in FIG. 5, corresponding to corner sections of the lower electrode film (20). When silicon nitride, for example, is deposited on the insulation film (30) by the sputtering process, insufficient covering or film quality defects tend to occur because of the steps created by the low electrode film (20). In addition to this drawback, the electric field is concentrated to this corner section, "A".
The upper insulation film (50) in FIG. 5 also tend to develop local insulation breakdown where it interfaces with the light emitting film (40). This is caused by the necessity of growing crystal grains (41) of ZnS or some other similar substance, as shown in FIG. 6, by applying heat treatment to improve the EL characteristics after the light emitting film (40) has been formed. This may create concaves and convexes of normally several hundred .ANG. on the surface (42) of the light emitting film (40), tending to cause mechanical distortion or cracks at points making contact with the tips of the convexes of the crystal grains (41) of the insulation film (30) and electric field concentration on these parts, thereby resulting in insulation breakdown.
In addition, if the EL indicating panel is used for color indication, and the light emitting film (40) is divided, rather than in a continuous form as in FIG. 5, into light emitting parts (40r, 40g, and 40b) as shown in FIG. 7 for indications in red, green and blue, corresponding to each picture element, the parts "B" and "C" in the figure of the insulation film (30 or 50) in the vicinity of the corners will turn out weak points resulting from insufficient covering, film quality degradation, and electric field concentration, ultimately leading to insulation breakdown.
If such insulation breakdown occurs, that section will suffer an indication defect. Fortunately, however, the electrode films (20) and (60) will not immediately be short circuited in the case of an EL indicating panel, and the panel will, therefore, remain usable even though the image quality may be somewhat degraded, while the defective area will be small and will not greatly expanded. However, because a defective point tends to gradually expand (although this will vary depending on insulating film material), with the number of defects increasing slowly, any defect in the insulation film can rapidly degrade the image quality and shorten the useful life of the EL indicating panel.
For these reasons, it is an object of the present invention to reduce weak points in the insulation films of the EL indicating panel, thereby preventing the degradation of the image quality and a corresponding decrease in service life.